Adding modern psychology to the Bible can be the recipe for nightmares in the wrong hands.

There’s some of that chill in the second half of Darren Aronofsky’s “Noah,” where ark fever (think cabin fever, multiplied by a thousand) and major PTSD blur the line between faith and madness. The twist, though not unrealistic, forms its own storm clouds over a movie that had previously been a powerful, moving fable about responsibility, devotion and the true nature of man.

In Aronofsky’s hands, the bones of the movie sidestep Christianity while (at least initially) remaining surprisingly true to the spirit of the source material. God becomes “The Creator,” and the stories of Adam and Eve and their children Cain, Abel and Seth are seen as part of the people’s history. Here’s where the branch-off happens, with the descendants of Cain taking over the land while the descendents of Seth care for creation and are taught to respect life. Unfortunately for the planet, the descendents of Seth are down to Noah and his family.

Most of the movie is filmed on empty, vaguely fantastical-looking landscapes that give the movie a subtle fantasy vibe, emphasized strongly by the movie’s rock monster-like take on fallen angels. The shift might be enough to smooth over some of the movie’s smaller doctrinal differences, which also borrow from Native American beliefs and bits that I’m pretty sure were just cooked up in Aronofsky’s head.

But even for the faithful, the early part of the movie can have a wrenching power. Its depiction of a world lost to darkness was easily the most striking I’ve ever seen, and Noah’s family shows some of the strain and hope that comes from clinging to a set of beliefs that everyone around you practically demands you reject. There were more than a few moments where I was deeply moved.

When the water came, however, other problems followed. For the sake of keeping major plot spoilers to myself I won’t reveal what happens, but the movie takes a solid side-detour into psychological horror that it doesn’t escape for what feels like a small eternity. The twist is well thought-out, and the intellectual in me insists that it’s a valid interpretation of a person’s possible state of mind in those circumstances. But it both rankles my faith and creates a very unpleasant patch of movie for the audience to be trapped in.

Russell Crowe is surprisingly good as Noah, successfully communicating power, danger and the wrenching internal weight of responsibility. Though Douglas Booth is nearly colorless as Shem, Noah’s oldest son, Logan Lerman brings a nuanced, deeply empathetic touch to the much more conflicted Ham. Jennifer Connelly is all quiet dignity as Noah’s wife, Naameh, while Emma Thompson holds up admirably well as Shem’s love interest Ila.

It’s impossible not to feel for these people, particularly in the movie’s darker moments. And even though the light returns – the world must continue, after all – it’s hard not to mourn for the version of the movie that has been irrevocably lost.